Scope of the Invention
This invention relates to a device for measuring with high sensitivity a diminished intensity of light pulses from such light emitters as organisms or organic compounds which are repeatedly being stimulated at high speed.
An important need exists for specifying the composition of an organic compound by precisely measuring a diminished intensity of light caused by fluorescence, or to locate focuses of an organism by precisely measuring a diminished intensity of light caused by fluorescence.
If the object, i.e., an organism or an organic compound is strongly stimulated, the nature of the object may change. Thus, an amount of stimulus sufficient to emit a measurable quantity of light cannot be obtained during fluorescence in many cases. It is well known that a streaking tube with a built-in micro-channel-plate can be used to measure a diminished intensity of light clocked at high speed.
Said streaking tube generates photoelectrons on its photoelectric layer responding to a diminished intensity of light. Its deflection electrode is used to deflect the electron beam of said photoelectrons, and its micro-channel-plate multiplies said photoelectrons so as to stimulate the phosphor layer located at the output of said micro-channel-plate. An intensity of multiplied light incident on the phosphor layer can thus be measured.
Sufficient brightness, in many cases, cannot be obtained on the phosphor layer even if such a device is used.
The inventors of the present invention tried to increase brightness by superposing a number of streaking images of light due to fluorescence on the phosphor layer when stimulus to the object being measured was synchronized with deflection of the streaking tube. This experiment, however, was unsuccessful.
The reason for the unsuccessful experiment was that the angle of collision of primary electrons with the dynode wall and the number of times the primary electrons collide with the dynode wall in the space between the channel inlet and outlet can vary.
The angle of collision of the primary electrons with the dynode wall affects the number of secondary electrons emitted, and the secondary electron multiplication factor increases as the angle of collision increases. The number of times the primary electrons collide affects the electron multiplication factor, and it is proportional to a certain power of the secondary electron multiplication factor defined as the frequency at which collisions occur.
The number of electrons issued from an arbitrary channel of the micro-channel-plate when a single electron is incident on that channel is distributed over a wide range of frequencies as shown at A in FIG. 1.
FIG. 1 shows that the frequency of occurrence of a fewer number of electrons emitted by collision of a single photon is higher than that of a larger number of electrons. It is well understood that the probability of occurrence of secondary electrons emitted by collision of a single photon decreases with the number of electrons.
If the streaking images obtained by a train of repetitive light pulses are superposed on the phosphor layer of said streaking tube consisting of a micro-channel-plate, a large variation can occur in the brightness of the streaking image caused by each light pulse on the phosphor layer, and a variation can also occur in the brightness of the superposed streaking images caused by repetitive light pulses. Unsatisfactory images can thus be obtained.
The quantum noise for N electrons is generally given by N.sup.1/2, and thus the S/N ratio is given by N.sup.1/2. One may think that the S/N ratio can be improved by the above superposition process; however, improvement is not obtained because noise is also generated by a variation in the multiplication factor of the micro-channel-plate mentioned above and the S/N ratio becomes greater than N.sup.1/2. Thus, the expected result could not be obtained by the above experiment.